Fabrication of a magnetoelastic torque sensor

ABSTRACT

A method of fabricating a magnetoelastic torque sensor includes plating a magnetoelastic material to a magnetically inert substrate, and its endowment with uniaxial magnetic anisotropy through the creation within the transducer element of stress anisotropy. The plating of magnetostrictive material to the magnetically inert substrate provides a less expensive torque element that exhibits desired levels of accuracy and reliability.

BACKGROUND OF THE INVENTION

This invention relates to a method of fabricating a magnetoelastictorque sensor. More particularly, this invention relates to fabricationof a magnetoelastic torque sensor having a transducer ring affixed to asubstrate in a plating process.

Conventional non-contact torque sensors include a magnetoelastic ringthat is supported on a shaft. A uniaxial, circumferential stressanisotropy is created in the magnetoelastic ring such that acircumferential magnetic anisotropy results, creating a predispositionwithin the transducer element toward a circumferential magnetic fieldwithin the magnetoelastic ring. The circumferential magnetic fieldbecomes distorted responsive to surface shear stress resulting from anapplied torque to the shaft. The amount and direction of magnetic fielddistortion is measured and provides a value utilized to provide thedesired torque measurements.

Conventional methods of fabricating a torque sensor include the pressingof a ring of positively-magnetostrictive material onto a magneticallyinert shaft. The shaft includes a taper such that a press fit of thering onto the shaft will produce the desired circumferential tension or“hoop stress”. The circumferential tension creates a magnetic easy axisand thereby facilitates the stability of the circumferential magneticfield. The press fit between the ring and the shaft is a limiting factorto the capability and accuracy of such torque sensors. Any slipping orrelative movement between the shaft and the ring distorts the actualreading of torque and causes a shift in the zero point of any torquemeasurement.

A known improvement over press fitting of rings onto a shaft includesthe use techniques such as thermal-spraying or kinetic metallization toapply a magnetoelastic material, particularly nickel, onto anon-ferromagnetic stainless steel substrate. The desired stressanisotropy for the magnetoelastic material is provided by applying anaxial load and heat to the substrate during the application of themagnetoelastic material. Once applied, the substrate is cooled and theaxial load released. This results in an axial tensile stress and acompressive circumferential hoop stress on the magnetoelastic materials,thereby producing the desired magnetic field. However, such afabrication method is difficult to control, requires the use of anexpensive grade of stainless steel for the substrate, and wastes much ofthe magnetoelastic material during the thermal spraying process.

Accordingly, it is desirable to develop a method of fabricating amagnetoelastic torque element that utilizes less expensive materials ina more reliable manner.

SUMMARY OF THE INVENTION

An example magnetoelastic torque sensor fabricated according to thisinvention includes a ring of magnetoelastic material that is plated to anon-ferromagnetic base shaft. The shaft, fabricated of non-magneticmaterials such as stainless steel, brass, or titanium, provides asubstrate that is magnetically inert for the magnetic transducermaterial. The substrate shaft is left in a non-stressed condition duringplating of the shaft. The plating process produces a very high ring toshaft adhesion strength. The high ring shaft adhesion strength improvesoperation of the torque sensor and reduces hysteresis and otheroccurrences that are common causes of sensor failure and degradation.

If the magnetoelastic material chosen for the transducer is apositively-magnetostrictive material, such as most steel alloys, thedesired stress anisotropy is one in which the transducer materialpossesses an axial compressive stress, and a circumferential tensilestress. Conversely, a negatively-magnetostrictive material, such asNickel or Cobalt and their alloys require an axial tensile stress and acircumferential compressive stress. Upon completion of the platingprocess, the substrate shaft is axially stressed in the appropriatedirection to a yield point that causes a permanent change in axialdimension of the shaft. The permanent axial deformation also causes achange in shaft diameter, which in turn causes a circumferential stresson the plated transducer ring creating requisite stress anisotropywithin the material.

Unlike the use of thermal application processes an electro platingprocess is a chemical plating process that yields a surface with highuniformity, low porosity, and high strength. The plating process createsvirtually no waste of ring material. Further, the ring to shaft adhesionstrength is much higher than in any other known torque fabricationtechnique.

Accordingly, the method according to this invention produces amagnetoelastic element for a torque sensor that has improved adhesionbetween the substrate and the magnetoelastic material ring at lower costand greater durability.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an example torque element according tothis invention.

FIG. 2 is a schematic view of a plating step of an example torqueelement according to this invention.

FIG. 3 is a schematic view of a fabrication step for an example torqueelement according to this invention.

FIG. 4 is a schematic view of another fabrication step according to thisinvention.

FIG. 5 is a schematic view of another example method of fabricating atorque element according to this invention.

FIG. 6 is a schematic view of another example method of fabricating atorque element according to this invention.

FIG. 7 is a schematic view of an example torque sensor according to thisinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a torque element 10 comprises a shaft 12 and amagnetoelastic nickel ring 14. The magnetoelastic ring 14 is plated tothe shaft 12 such that torque exerted on the shaft 12 causes torsionalstresses on the magnetoelastic ring 14. The magnetoelastic ring 14includes a stress anisotropy such that a tensile stress exists in theaxial direction, and a compressive stress is circumferentially disposedabout the shaft 12.

The circumferential stress or hoop stress encourages formation of amagnetic field having circumferential lines of magnetic flux. Asappreciated, the circumferential lines of magnetic flux distortresponsive to a torque applied to the shaft 12. The distortion ismeasured to determine torque. The magnetoelastic ring 14 comprises anickel material plated on the shaft 12. The shaft 12 is fabricated froma non-magnetic grade stainless steel.

Referring to FIGS. 2 and 3, the inventive method includes first platingthe magnetoelastic ring 14 onto the shaft 12. During the plating step,no load is placed on the shaft 12 and therefore the stress within themagnetoelastic ring 14 is isotropic and not disposed in the desiredcircumferential orientation. In order to operate as desired, themagnetoelastic ring 14 should be of higher yield strength than theunderlying substrate. The example magnetoelastic ring 14 comprises anickel-plating material that typically includes yield strength on theorder of 60,000 to 80,000 psi. The relationship between the yieldstrength of the plating material and the shaft material determines theselection of the shaft material. Accordingly, using nickel-platingmaterial provides for a shaft 12 having a lower yield strength, which inturn provides for the use of a more economical grade of stainless steelalloy. The example shaft is fabricated form a 316-grade stainless stealalloy that in an annealed condition includes yield strength ofapproximately 35,000 psi. As should be understood, other grades andcombinations of materials with desirable characteristics may also beutilized.

As understood, circumferential stress within the magnetoelastic ring 14provides anistropic magnetic properties within the ring 14. The magneticproperties generated by the hoop stress are such that less resistance toa magnetic field is provided in the circumferential direction. The asplated nickel material is of a uniform thickness and porosity and doesnot include the desired circumferential stress. Circumferential stressmust therefore be introduced to the ring 14 to induce the stressanisotropy.

Once plated, an axial tension load, schematically indicated at 32, isexerted on the shaft 12 until plastic deformation occurs. The shaft 12after plating is of a length 20 and then is plastically deformed to alength 22. The extended length 22 is accompanied by a reduction indiameter from an original diameter 24 to a reduced diameter 26. Thereduction in diameter 26 of the shaft 12 induces a compressive stress,schematically indicated at 30, on the magnetoelastic ring 14 thatcreates the desired magnetic anisotropy circumferentially about theshaft 12.

The desired anisotropy circumferentially disposed within the ring 14about the shaft 12 creates the desired magnetic easy-axis 28 in thecircumferential direction, facilitating a circumferential magnetization.Application of torque to the shaft 12 distorts this magnetic field 28 ina helical direction away from the circumferential direction and towardsa more axial direction. Measurement of this shift in the magnetic fieldprovides for the accurate determination of torque exerted on the shaft12. As appreciated, plastic deformation in the axial direction isaccompanied by deformation and reduction in diameter 26. This reductionin diameter produces the desired compressive stresses on the platednickel material.

Referring to FIG. 4, another method according to this invention includesthe initial step of plating magnetoelastic material 44 to a shaft 42.The shaft 42 serves as a substrate for the magnetoelastic material thatforms the ring 44. In this example method, the ring 44 is illustrated asbeing plated within a step or reduced diameter region 45 of the shaft42. Although, in some instances a reduced diameter region 45 providing areduced cross-section may be preferable it is not required, the ring 44may be plated on the greatest diameter of the shaft 42. The shaft 42 mayhave a single common continuous uniform diameter and the ring 44 may beplated onto that diameter in a desired location.

Further, the ring 44 may be of any axial width as is required for theapplication specific requirement. Typically, the width of the ring 44will correspond with a pick up device utilized to sense changes in themagnetic field within the magnetoelastic material of the ring 44.

It is understood in the field of electroplating that the electrolyticprocess of plating material to a substrate results in a natural statewhere the plated material produces a significant stress in itself. Thissignificant stress is in many instances overcome by specific applicationand process parameters that are tailored to reduce this induced stress.However, an example method and application of plating for amagnetoelastic torque element, such tensile stress is favorable as acomponent of the desired stress anisotropy. Accordingly, it is possiblewith the proper selection of substrate and magnetoelastic material toinstill the required tensile hoop stresses in the ring material 44simply through the plating processes.

Preferably, in the processes illustrated in FIG. 4 the underlying shaft42 is magnetically inert stainless steel material such as the stainlesssteel alloy 316. The magnetoelastic material plated on this material isa nickel iron alloy that instills the desired tensile stresses by thecontrolled conditions of the plating process. In some instances theinduced tensile stress can be significant up to 100,000 psi.

Accordingly, the process illustrated in FIG. 4 comprises the steps ofselecting a substrate material compatible with a ring plating material.The ring plating material comprises a nickel iron mix. The nickel ironmix is formulated in such a combination that the plating process itselfwill create a desired level of tensile stress in the ring 44. Once theplating has completed to form the ring 44 the required tensile stresseson the ring 44 create the desired magnetic field orientationcircumferentially within the ring 44 around the shaft 42. Such a processis desirable if the material selections are within the applicationspecific parameters that require no additional processing once the ringmaterial 44 has been plated onto the shaft 42.

Referring to FIG. 5, another example method according to this inventionis illustrated and includes the initial step of plating ring material 64onto a shaft 62. The ring material 64 is plated by applying nickel ironor nickel material as indicated at 66 onto the shaft 62. During theplating process, the shaft 62 is placed under an axial load as isindicated by arrows 68. The axial load 68 is applying a tensile stresson the shaft 62 that does not create a plastic deformation.

Once the plating process is complete, the axial load 68 is released andthe shaft 62 will substantially regain a substantially original lengthand width. The stretched length 74 will be reduced to a substantiallynatural length 75. As appreciated, when an axial tension load is placedon the shaft 62, the diameter 76 is reduced over that of a non-stresseddiameter 77. Release of the axial load 68 causes the diameter to movefrom the stretched state 76 to the relaxed state 77. The relaxed state77 is greater than the stretched state such that tensile stresses 72 areplaced on the ring 64 in a desired manner. These tensile stresses createthe desired circumferential magnetic easy access desired to provide thedesired direction of the magnetic field created in the nickel iron ring.

Referring to FIG. 6, another example method of fabricating amagnetoelastic element for a torque sensor is illustrated and includesthe initial step of plating a nickel-iron material 86 onto a shaft 82 toform a ring 84. The shaft 82 in this example method is not placed underany load during the nickel-iron-plating process. The nickel-iron-platingprocess induces certain stresses within the ring 84 and the shaft 82that may not provide the desired directional anisotropy of ring 84conducive to the desired orientation of the magnetic field.

Accordingly, deformation of the underlying shaft 82 is utilized toinduce stresses on the plated ring 84 and thereby produce the desiredcircumferential magnetic field orientation. In this process, an axialload 96 is placed on the shaft 82. The axial load is a compressing loadresulting in a decrease from a length 88 to a length 90 and an increasein a diameter 92 to a diameter 94. The compressive stresses are applieduntil plastic deformation occurs such that the desired stresses on thering material 84 will remain after the load 96 is removed.

The magnetic field generated by the torque element is proportional tothe magnetostrictive properties of the ring material. Additionally, themagnetocrystalline properties also affect the amount of hysteresis thatwill be observed within the sensor under torque. Electroplating a nickeliron material has essentially a nanocrystalline morphology thus is amagnetocrystalline anisotropy that is extremely small. Themagnetostriction of the material is determined by the alloy percentagesof the ring. The preferred material configuration of the magnetoelasticmaterial includes 45-55% nickel. Such benefits are provided by theplating process utilized by the method of this invention and provide forthe fabrication of the favorably reliable and accurate torque sensor.

Referring to FIG. 7, a torque sensor 100 fabricated according to thismethod includes the magnetoelastic torque element 102 that is platedwith a ring material 108. The ring material 108 provides acircumferential easy magnetic access such that a magnetic field 110 isdisposed circumferentially about the shaft 102 within the ring 108. Amagnetometer 104 is disposed around the shaft 102 and specifically aboutthe ring 108. A torque 112 applied to the shaft 102 will distort themagnetic field 110 which will in turn be detected by the magnetometer104. The changes in the magnetic field generate a variation in a voltagegenerated by the magnetometer 104 that is detected by a controller 106and converted in a known manner to provide measurements of the degree oftorque 112 applied to the shaft 102.

The example methods disclosed herein provide advantages known to priorart application by providing great reductions in cost of the applicationof ring material and increasing performance capability of the torquesensor itself. The plating of transducer material to a shaft increasesthe maximum torque to which the sensor can be utilized as the adhesionin the plating and the shaft is superior to other methods utilized inprior art methods.

Although a preferred embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

1. A method of forming a magnetoelastic element comprising the steps of:a) plating a ring of magnetoelastic material onto a portion of asubstrate; and b) plastically deforming the substrate after plating toendow the magnetoelastic material with a desired stress anisotropy. 2.The method as recited in claim 1, wherein said step b) includes applyingan axial load on the substrate.
 3. The method as recited in claim 2,wherein said axial load applies a tensile force on said substrate. 4.The method as recited in claim 2, wherein said axial load applies acompressive force on said substrate.
 5. The method as recited in claim3, wherein said axial load causes a permanent elongation of thesubstrate.
 6. The method as recited in claim 1, wherein said substratecomprises a stainless steel material.
 7. The method as recited in claim1, wherein said magnetoelastic material comprises at least a portion ofnickel material.
 8. The method as recited in claim 1, wherein saidsubstrate comprises a notched diameter within which said magnetoelasticmaterial is plated.
 9. The method as recited in claim 1, wherein saiddesired stress anisotropy comprises circumferentially orientatedstresses in said magnetoelastic material to generate a magnetic easyaxis circumferentially about said substrate.
 10. A method of forming amagnetoelastic torque sensor assembly comprising: a) plating a ring of amaterial comprising nickel about a circumference of a substrate; and b)plastically deforming the substrate to generate a desired stress withinsaid ring that provides for the generation of a magnetic filed in adesired orientation.
 11. The method as recited in claim 10, wherein saidring of material is comprises nickel and iron.
 12. The method as recitedin claim 10, wherein said substrate includes a reduced cross-section inan area proximate to the ring of material.
 13. The method as recited inclaim 10, including the step of plastically deforming the substrateprior to said plating step.
 14. The method as recited in claim 13,wherein the step of plastically deforming the substrate comprisesexerting an axial tension load on the substrate.
 15. The method asrecited in claim 13, wherein the step of plastically deforming thesubstrate comprises exerting an axial compressive load on the substrate.16. A torque sensor assembly comprising: a substrate for encountering atorque load; a ring fabricated from a magnetoelastic material plated tosaid substrate, wherein said magnetoelastic material includes lowmagnetic resistance in a circumferential orientation about saidsubstrate; and a magnetic sensing device for detecting distortion of amagnetic field induced by the application of torque to said substrate.17. The assembly as recited in claim 16, wherein said magnetoelasticmaterial comprises nickel plated to circumferentially onto saidsubstrate.
 18. The assembly as recited in claim 16, wherein saidsubstrate is plastically deformed to instill the desired orientation ofstress within said ring.
 19. The assembly as recited in claim 18,wherein said substrate is plastically deformed to change across-sectional area proximate to said ring to instill a desired stressanisotropy within said magnetoelastic material.